1. Field of the Invention
This invention relates to an air gun for generating an acoustic pulse train in a body of water. 2. Description of Related Prior Art
In one method of land seismic exploration, described, for example in U.S. Pat. No. 3,688,124, a sweep or chirp signal, generated by a vibrator, is injected into the ground. The chirp-signal, also referred to as a swept-frequency signal, is a unique wave train whose frequency pattern is non-repetitive during a period of time that is at least twice as long as the maximum reflection travel time. The reflected signals, which are detected by geophones laid on the surface of the earth, are recorded on magnetic tape. The recorded reflected signals are cross-correlated with a replica of the original chirp-signal to produce a correlogram.
Because of the great commercial success of vibrators on land, workers in the seismic art have for many years attempted to adapt land vibrators for use at sea. But serious problems were encountered. The marine vibrators were hydraulically or pneumatically operated. They generally took the form of large bellow or vibrating diaphragms many feet in diameter. A typical such device is disclosed in U.S. Pat. No. 3,394,775. Because marine vibrators were large and bulky, they were at first installed in a well, cut in the hull of a ship. The constant vibration from the vibrator caused fatigue-cracks throughout the ship's structure. Later, the vibrators were trailed in the water alongside the ship from booms, as described in U.S. Pat. No. 3,452,327. The heavy, unwieldy vibrators, with their multiplicity of hydraulic and/or pneumatic control hoses, required complex handling gear on the support ship. Furthermore, since they were designed primarily for seismic efficiency, the vibrators were hydrodynamically unstable, they trailed erratically when under tow and required a large towing force.
The seismic chirp-signal generated during a recording cycle by the vibrator discussed above is a continuous oscillatory wave train of 5 to 15 seconds duration. The initial and final frequencies of the signal lie within the useful seismic spectrum, usually within the range of 3 to 100 Hz. A typical chirp-signal, for example, might start at 5 Hz and terminate at 40 Hz. Many other frequency ranges are in common use.
It is possible to generate and analogous signal in the form of a discrete pulse train. The interval between successive pulses may be likened to the period of a wavelet, the period being the reciprocal of the frequency. In a marine environment, a relatively low-frequency discrete pulse train can be generated by a gas or air gun of any type well known to the art. One such method is described in an article beginning on page 673 of the August, 1973 issue of Geophysics. As pointed out by the author however, state-of-the-art air or gas guns are limited to equivalent frequencies of up to 7 Hz. Such a restricted, relatively low-frequency range for the seismic spectrum is unsuitable for general use with accepted chirp-signal correlation techniques.
Typical air guns used in seismic exploration, are described in U.S. Pat. Nos. 3,638,752, 3,249,177 and 3,276,534. All of these air guns are "one shot" sound sources. That is, for each recording cycle of 5 to 15 seconds, the gun is fired but once.
Other pulse-generating devices, such as a pulsed sonar or marine sparker, are available. However, pulsed-sonar devices do not have sufficient output power to penetrate the earth to a depth that is commercially useful in oil exploration. Because of practical physical limitations of the input power versus capacitor charging rate, the firing repetition rate or equivalent frequency of a sparker is, at best, only a few Hertz. A typical range is 1 to 4 Hz.
In application Ser. No. 647,549, now U.S. Pat. No. 4,049,078, issued to the assignee of this application, a "Marine Acoustic Pulse Train Generator" is disclosed. Basically, in the device of the earlier application a sleeve valve is slidingly mounted within the barrel of the air gun. Driven by a rotary cam, through a mechanical linkage, the sleeve valve is reciprocatingly driven to open and close an exhaust port in the gun barrel. Although the gun accomplished its purpose of generating a pulse train at a desired frequency in the water, the complexity of the cam and linkage mechanism rendered the gun difficult to maintain. An object of the present invention is to provide a somewhat simpler, rapid-fire air gun.